Joint replacement surgery is quite common and enables many individuals to function normally when otherwise it would not be possible to do so. Artificial joints are normally composed of metallic and/or ceramic components that are fixed to existing bone.
Knee arthroplasty is a well known surgical procedure by which a diseased and/or damaged natural knee joint is replaced with a prosthetic knee joint. Typical knee prostheses include a femoral component, a patella component, a tibial tray or plateau, and a tibial bearing member. The femoral component generally includes a pair of laterally spaced apart condylar portions, the inferior or distal surfaces of which articulate with complementary condylar elements formed in a tibial bearing component.
In a properly functioning artificial knee joint, the condylar portions of the femoral component must slide and roll freely over the articulation surface formed by the condylar elements of the tibial bearing member. Natural friction within a replaced, artificial joint can lead to the development of wear debris in which minute particles of debris (e.g., metal or plastic from the prosthesis) become dislodged and migrate within the joint. The phenomenon of wear debris within artificial joints is a serious problem that can inhibit the proper mechanical functioning of the joint. Moreover, wear debris can lead to osteolysis and bone deterioration. When wear debris develops within an artificial joint, surgical removal of the debris or subsequent replacement of the artificial joint is often necessary.
During normal usage of a properly implanted prosthetic knee joint, load and stress are placed on the tibial bearing member. The tibial bearing member is typically made of an ultrahigh molecular weight polyethylene (UHMWPE), and friction, continuous cycling and stress can cause some erosion and/or fracture of the tibial bearing member, thus leading to wear debris. The risk of wear debris can be even greater during malalignment of an artificial knee joint, which can result from normal usage or from imperfect and/or inaccurate implantation of the prosthesis within a patient. During malalignment the load upon the tibial bearing member is not evenly distributed. Instead, excess load is placed on certain areas of the tibial bearing member. This uneven distribution of load (or edge loading) can accelerate the development of wear debris. Contact stresses on the tibial bearing member increase substantially with malalignment of the joint, thus increasing the risk that wear debris will develop when a prosthetic knee joint is subjected to malalignment conditions.
Joint replacement surgery obviously requires a tremendous degree of precision to ensure that prosthetic components are properly sized, implanted, and aligned. Imperfect sizing, implantation and alignment can lead to inadequate performance of the knee joint as well as to the presence of high contact stresses in certain areas of the prosthesis, thus leading to the possible development of wear debris.
The anatomy of patients who undergo knee arthroplasty is widely variable and can lead to difficulty in matching the standard sized prosthetic components that form a prosthetic joint. Many prosthetic components are manufactured such that similarly sized components must be used together and implanted within a patient when replacing a natural joint. That is, the femoral component, tibial bearing member, and tibial plateau that form the artificial knee joint must normally be of a matched size. If the components are not size-matched, inappropriate edge loading may develop and accelerate wear.
FIG. 1 illustrates three components found in a typical knee joint prosthesis 10. A femoral component 12 includes a superior surface 14 which is mountable within the distal end of a patient's femur and an inferior articulation surface 16. The articulation surface 16 includes adjacent condyles 18. The knee prosthesis 10 also includes a tibial tray or plateau 20 which includes a distally extending stem 22 that is mountable within the tibia of a patient. The proximal end of the tibial tray 20 includes a recessed region 24 within which a tibial bearing member 26 is mounted in a mechanical fit.
Tibial bearing member 26 includes a distal surface 30 mountable within the recessed region 24 of the proximal end of a tibial tray 20 plateau 24. The proximal face of tibial bearing member 26 forms articulation surfaces 28 that engage and articulate with the articulation surfaces 16 of femoral component 12. The articulation surfaces 28 of the tibial bearing member 26 are configured to correspond to the condyles 18 of the femoral component 12.
The articulation surface 16 of femoral component 12 and the articulation surfaces 28 of tibial bearing member 26 are configured such that the contact area is maximized. The greatest contact area is achieved in conditions of perfect alignment throughout the range of motion of the knee joint, and in certain conditions of malalignment, including varus-valgus lift and internal-external rotation. The ability to achieve a large contact area between the articulating surfaces is significant because contact stress on the prosthesis components is minimized, particularly the tibial bearing member. Most standard tibial bearing members are manufactured of polymeric materials, such as ultra-high molecular weight polyethylene (UHMWPE), ceramic or metal. Where loads are unevenly distributed or concentrated across the tibial bearing member during use of an artificial knee joint, edge loading can develop. Edge loading leads to the development of higher contact stresses in certain parts of the prosthesis which, in turn, can cause wear of the articulating surfaces. Debris resulting from this wear can develop within the joint, sometimes leading to osteolysis.
More significantly, undue bearing wear can result in conditions requiring that the joint endoprosthesis be removed and replaced in a revision procedure. Accordingly, early determination of unacceptable wear conditions is critical. Misalignment of the joint prosthesis components can be detected during the implantation procedure and during rehabilitation of the new joint. Various measurements and templates can evaluate proper positioning and spacing of the components.
Another important indicator of proper or improper alignment is the distribution and transfer of loads across the prosthesis. In particular, loads experienced by the tibial tray 20 can provide the earliest indication of bad joint “fit”. In order to evaluate these loads, telemetric implant components have been developed, such as the dual tray telemetric implant described in U.S. Pat. No. 5,360,016 (“016 patent”), the disclosure of which is incorporated herein by reference. A force transducer is incorporated into the proximal tibial component of the implant. The force transducer uses strain gages to generate output signals indicative of force measurement data that can be used to assess pressure differences across the surface of the tibial tray which may be indicative of an improperly aligned implant.
Another telemetric implant is embodied in a tibial component 40 depicted in FIGS. 2 and 3. The tibial component 40 includes a stem 42 configured to be engaged within the tibia. A tibial tray 44 is mounted on the stem, and includes a cover plate 46 that is directly attached to the stem. A lower plate 48 is mounted on the cover plate, while an upper plate 50 is supported on the lower plate by a plurality of support posts 52. As best seen in FIG. 3, the lower plate 48 includes a perimeter wall 54 configured to engage the cover plate 46. Fasteners (not shown) are used to fasten the two plates together.
The lower plate defines a plurality of transducer cavities 56, each corresponding to a support post. The base of each cavity defines a diaphragm 63 to which a corresponding support post 52 is attached or integrally formed. The support posts are preferably integral with the lower plate 48 and the upper plate 50 but are configured to separate the two plates by a gap 53. Load applied to the upper plate 50 is transmitted through the support posts 52 to the integral diaphragms 63 which flex in relation to the transmitted load.
In order to measure the deflection of these diaphragms, a force sensing element is disposed within each transducer cavity. More specifically, the force sensing elements include an array of strain gages that are affixed to the diaphragm at the base of each transducer cavity 63. As shown in FIG. 3, each strain gage array includes four radially inner strain gages 67 and four radially outer gages 69 disposed at the four compass points around the cavity. More specifically, the strain gages are arranged in planes that are at 90 degrees or 180 degrees to the sagittal and/or lateral planes of the knee joint prosthesi.
The strain gages include wiring 71 that passes through wiring channels 60 and 61 to a centrally located circuitry cavity 58. A processing circuit board 73 is disposed within this cavity and includes electrical components and/or integrated circuits adapted to process the output of the strain gages and facilitate translation of that output into load information. In some implants, such as the force transducer disclosed in the '016 patent incorporated above, the circuit board 73 serves to condition the strain gage signals and to provide a wiring harness for connection to an external processor or computer. In other implants, the circuit board 73 prepares the strain gage signals for transmission by a transmission device. In some implants, the circuit board includes a telemetry device and a power supply. In other implants, the stem 42 (FIG. 2) carries a telemetry device 75 and associated power source 77 adapted to transmit the strain gage output signals to an external processor where the signals are evaluated.
In the telemetric tibial component 40 shown in FIG. 3, a no-load post 65 projects from the diaphragm 63 between the inner strain gages 67. It can be appreciated that the no load posts 65 are essentially co-linear with the support posts 52, although the two sets of posts reside on opposite sides of the diaphragm 63 of each transducer cavity 56. The no-load posts are believed to promote a circumferentially symmetric strain pattern within each cavity.
The introduction of telemetric implants has provided a means for evaluating the loads actually experienced by an endoprosthesis. This evaluation can occur in real-time as the joint is exercised and loaded. However, since the primary function of the implant is to serve as a prosthetic joint, and not simply as a data transmission device, the implant must be able to withstand joint loads without failure. Load is transmitted from the femur to the tibia through the large articulating surface areas of the condylar surfaces 16 and the bearing surfaces 28. However, once the load reaches the tibial tray, such as the tibial tray 44, the force is transmitted through four support posts 52 into the tibia. Therefore, it can be appreciated that the strength of these posts is critical to the strength of the implant.
In conflict with need for structural strength is the need to generate sufficient strain in the diaphragms 63 such that a measurable strain differential may be detected between the strain gages 67 and 69. The ability to accurately measure the forces transmitted across the joint space is enhanced as the magnitude of the strain differential increases. The trade-off for a stronger implant has been a reduction in diaphragm strain and a sacrifice in accuracy of the load measurement. The introduction of the no-load posts 65 is an effort to recapture some accuracy in the load measurement capabilities of the strain gage arrays. There remains room for improvement in both the strength of the telemetric implant component as well as the ability of the transducer component to provide a true measure of the loads transmitted across the joint.